The national high and low temperature
for the contiguous 48 states are compiled by the National Weather Service's
Hydrometeorological Prediction Center in Silver Spring, Md., says Kevin McCarthy,
Deputy Director of HPC. The national high and low are determined from information
provided by local National Weather Service offices.

McCarthy says the guidelines for temperature eligibility
are:

High and low temperature must be from the conterminous
U.S. (excluding Hawaii and Alaska)

The elevation must not be above 8,500 feet.

The location must have a population of more than 1,000.

The USATODAY.com Weather team adds temperature data for Alaska and Hawaii,
when one of these states has the highest or lowest for the entire USA. We
use data from the temperature and precipitation summaries transmitted from
the Anchorage, Alaska and Honolulu weather service offices. When Alaska has
the coldest U.S. temperature or Hawaii the warmest, we also list the coldest
or warmest for the 48 contiguous states because we know that many weather
buffs track these figures.